1. Field of the Invention
The present invention relates to a method for constructing an offshore platform structure having inclined supporting legs.
2. Prior Arts
Concrete platform structures having inclined supporting legs are commonly known. U.S. Pat. No. 4,043,138 discloses an open frame offshore foundation structure of concrete to be firmly but removably located on a sea floor. The structure comprises a lower set of hollow columns inclined towards each other in direction upwards, a ballastable intertie box structure rigidly receiving the tops of said inclined columns and an upper vertical column unit, the lower ends of which are rigidly connected to the intertie box structure and the upper ends of which are intended to support a deck superstructure in position above the sea level. Both the lower set of inclined columns and the upper vertical columns are cast using slip form, the lower set of inclined columns being cast using inclined slip forms.
Inclined slip forming is substantially more complicated and expensive than casting vertical columns using slip forms. Further, when using the conventional slip forming technique for casting inclined columns it is not possible to vary the diameter of the column during casting. It should further be appreciated that in the conventional slip forming technique for casting inclined legs, the inclination cannot exceed approximately 20.degree. from the vertical. In particular, but not exclusively, such types of platform structures are particularly suitable for depths exceeding 100 meters. As platform structures in general are designed for increasing depths, the lengths of the inclined legs are increased correspondingly. However, since the legs according to the conventional method of construction are rigidly affixed to the foundation even during the construction stage, temporarily arranged provisional supports or stiffeners must be used during the construction phase in order to reduce dynamic stresses imposed on the raft to an acceptable level.
Since the inclination of the legs is limited, the sectional forces appearing in any cross-section of the legs when installed on the site will be excessive unless ballast is added to improve the stability of the structure. Consequently, both construction and installation work become complicated and more costly. If the inclination is increased, the area of the dry dock must be increased correspondingly. The same considerations apply to platform structures designed for great depths.